Zinc phosphate coatings are deposited on metals by chemical reaction with an acidic solution of zinc dihydrogen phosphate. The zinc phosphate coating by itself provides a measure of protection against atmospheric and other sources or corrosion. More importantly, the zinc phosphate coatings are combined with an organic top coat such as a paint or lacquer to provide additional corrosion protection and an attractive finish. Heavy deposits of zinc phosphate on metals are also used as a carrier for soaps and other lubricants for deep drawing of metals.
A chronic problem associated with the formation of zinc phosphate coatings on metal is the build up of a hard rock-like scale on heating elements, headers, nozzles and the tank used to hold the phosphating solution. If spray nozzles are being used to apply the phosphating solution the scale build-up decreases the amount of spray, changes the spray patterns and gives non-uniform coatings. Excessive build-up on heating coils acts as an insulating medium and leads to poor heat transfer and eventual shut-down of the phosphating operation. Cleaning of the heating coils is a costly time consuming operation and damage to the coils occurs frequently.
To remove this hard scale and sludge build-up is a major maintenance problem which requires shutting down and phosphating line at frequent intervals. The hard rock-like scale is quite difficult to remove from the equipment and may require use of air hammers and/or aqueous solutions of mineral acids to assist in the scale-removal operation.
In addition to reducing the amount of hard sludge formed on the phosphating equipment the ideal sludge reducer must be compatible with the zinc phosphate concentrates used to prepare and replenish the phosphating baths so that the sludge reducer can be added as a part of the phosphating chemicals. This compatibility has the advantage of insuring that a proper concentration of sludge reducer is always in the phosphating bath, and in addition it makes unnecessary a separate chemical addition and chemical control to maintain the proper concentration of the sludge reducer in the phosphating bath. It is, of course, essential that the sludge reducer does not interfere with the weight or quality of the phosphate coating which is deposited on the metal work piece.
We have now discovered that if we add a small amount of a dialkyl triamine pentakis methylene phosphonic acid or its alkali metal or ammonium salt to the zinc phosphating solution that the phosphate scale and sludge formed as a result of the phosphating reaction is greatly decreased in amount, is non-hardening and is easily removed from the equipment by rinsing with water. The terms scale and sludge have the same meaning and are used interchangeably throughout this specification.